Under certain conditions, Naegleria fowleri can develop flagella—threadlike structures that enable it to rapidly move around and look for more favorable conditions. When people swim in warm freshwater during the summer, water contaminated with the moving amoeba can be forced up the nose and into the brain.
This causes headache, stiff neck, and vomiting, which progresses to more serious symptoms. Between exposure and onset, infection generally results in a coma and death after around five days.
Since 1962, there have been 128 cases of Naegleria fowleri [infection] and only one survivor...
For bonus horror, note this information and image for your nightmares: "The feeding structures of the amoeba Naegleria fowleri have a face-like appearance." See?
(thanks to Andrea Lobel Shainblum)